Guidance sections employing a conventional null-seeker design employ a rate-control loop to maintain the target at the center of the radar antenna gimbal axis (the boresight). The boresight error (BSE) is a position error that represents the difference between the perceived target position and the true target position. The guidance section is conventionally configured to correct for the BSE to increase the probability of guidance (Pg) that a missile may achieve for various threat and trajectory scenarios. The configuration may be optimized to maintain an optimum guidance loop bandwidth and BSE is the independent variable. One difficulty is the creation of BSE correction factors related to manufacturing test data with tactical performance to achieve a tactical configuration of optimum guidance loop bandwidth to sustain an optimum Pg.
There are three dominant issues related to radome errors that may significantly affect the Pg. The refraction properties of the radome may produce noise that is added to a guidance control signal within the guidance control loop. One issue that degrades the Pg is the increased error in estimating a target's instantaneous position. A second issue that degrades the Pg is the mean and variance of the noise that adaptive filters of the guidance loop attenuate as these independent variables determine the adaptive filter bandwidth. A third issue is that the radome refracts the incoming radar signals within the field-of-regard (FOR) of the radome. A target random walk within the FOR produces a sample subset of the random noise within the entire FOR. This subset may be a non-probability sample (i.e., not having the same mean and variance as the entire FOR error set).
Noise within the target velocity bandwidth of the guidance control loop is retained by the guidance loop as it is provided a feedback path (e.g., through by missile body motion coupling). These retained errors may induce undesirable trajectory changes in the flight path resulting in lost energy and thus diminishing the Pg.
Radome refraction errors may be characterized as a topographical error surface associated with the FOR position. A corresponding BSE correction table may be used to reduce instantaneous position errors. The spatial resolution within the FOR of the error correction table may determine a smoothing function of the measured error. The correction of errors between entries in the table is conventionally achieved by estimation using linear interpolation to estimate the amount of BSE between table entries. These conventional correction tables are created with the at least three assumptions linked to the three issues described previously and a fourth assumption related to standard signal processing algorithms. The first assumption is that the residual error (noise) that is outside the target signal bandwidth will be attenuated by the guidance system noise filters. The second assumption is that all noise will have a Gaussian or broadband mean and variance. The third assumption is that the random walk of the target within the FOR will generate noise that is a true subset of all noise within the FOR. The fourth assumption is that given the measured error surface representing the error within the FOR is dominated by a sinusoidal bias along a radial vector from bore-sight, it is reasonable to use the Fast Fourier Transform to generate a noise power spectral density plot that relates power to the spatial wavelength of the bias. These assumptions may require a special solution dependent upon materials and manufacturing processes, not an independent solution derived from the implementation factors that can prescribe the system performance requirements.
One tactical factor associated with conventional correction tables is the addition of bias which results in a non-monotonic correction function. This nonlinearity compounded with the assumptions taken significantly impacts the statistics of the residual noise. This may change the mean and variance of the tactical noise subset resulting in a degraded Pg.
These conventional correction tables further depend on the material and manufacturing processes which generally conforms to pyroceramic radomes. These conventional correction tables are generally not suitable for use with composite radomes because the change in materials and processes may significantly impact the statistics of the refraction-induced noise. Consequently the difference between a tactical noise Probability Density Function (PDF) and the FOR PDF may be large enough to increase the error in estimation of the instantaneous target position because of the reduced bandwidth of the adaptive filter in response to the noise variance for which a conventional correction table does not account.
Thus, there are general needs for improved methods for compensating for BSE in missiles, particularly for missiles that use composite radomes. There are also general needs for guidance sections with improved BSE compensation.